From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0F024C433E4 for ; Mon, 17 Aug 2020 03:04:41 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 815B92087D for ; Mon, 17 Aug 2020 03:04:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 815B92087D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id AEC936B0003; Sun, 16 Aug 2020 23:04:39 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A9CD56B0005; Sun, 16 Aug 2020 23:04:39 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9B1FF6B0006; Sun, 16 Aug 2020 23:04:39 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0251.hostedemail.com [216.40.44.251]) by kanga.kvack.org (Postfix) with ESMTP id 864456B0003 for ; Sun, 16 Aug 2020 23:04:39 -0400 (EDT) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 42B043655 for ; Mon, 17 Aug 2020 03:04:39 +0000 (UTC) X-FDA: 77158567878.18.show02_2c0f5c627013 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin18.hostedemail.com (Postfix) with ESMTP id 1AC75100EC675 for ; Mon, 17 Aug 2020 03:04:39 +0000 (UTC) X-HE-Tag: show02_2c0f5c627013 X-Filterd-Recvd-Size: 5874 Received: from out30-42.freemail.mail.aliyun.com (out30-42.freemail.mail.aliyun.com [115.124.30.42]) by imf19.hostedemail.com (Postfix) with ESMTP for ; Mon, 17 Aug 2020 03:04:37 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R801e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07488;MF=richard.weiyang@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0U5wd.eG_1597633465; Received: from localhost(mailfrom:richard.weiyang@linux.alibaba.com fp:SMTPD_---0U5wd.eG_1597633465) by smtp.aliyun-inc.com(127.0.0.1); Mon, 17 Aug 2020 11:04:26 +0800 Date: Mon, 17 Aug 2020 11:04:25 +0800 From: Wei Yang To: Michal Hocko Cc: Mike Kravetz , Baoquan He , Wei Yang , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 10/10] mm/hugetlb: not necessary to abuse temporary page to workaround the nasty free_huge_page Message-ID: <20200817030425.GA25240@L-31X9LVDL-1304.local> Reply-To: Wei Yang References: <20200807091251.12129-1-richard.weiyang@linux.alibaba.com> <20200807091251.12129-11-richard.weiyang@linux.alibaba.com> <20200810021737.GV14854@MiWiFi-R3L-srv> <129cc03e-c6d5-24f8-2f3c-f5a3cc821e76@oracle.com> <20200811015148.GA10792@MiWiFi-R3L-srv> <20200811065406.GC4793@dhcp22.suse.cz> <20200813114638.GJ9477@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200813114638.GJ9477@dhcp22.suse.cz> X-Rspamd-Queue-Id: 1AC75100EC675 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 X-Bogosity: Ham, tests=bogofilter, spamicity=0.001234, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Aug 13, 2020 at 01:46:38PM +0200, Michal Hocko wrote: >On Tue 11-08-20 14:43:28, Mike Kravetz wrote: >> On 8/10/20 11:54 PM, Michal Hocko wrote: >> > >> > I have managed to forgot all the juicy details since I have made that >> > change. All that remains is that the surplus pages accounting was quite >> > tricky and back then I didn't figure out a simpler method that would >> > achieve the consistent look at those counters. As mentioned above I >> > suspect this could lead to pre-mature allocation failures while the >> > migration is ongoing. >> >> It is likely lost in the e-mail thread, but the suggested change was to >> alloc_surplus_huge_page(). The code which allocates the migration target >> (alloc_migrate_huge_page) will not be changed. So, this should not be >> an issue. > >OK, I've missed that obviously. > >> > Sure quite unlikely to happen and the race window >> > is likely very small. Maybe this is even acceptable but I would strongly >> > recommend to have all this thinking documented in the changelog. >> >> I wrote down a description of what happens in the two different approaches >> "temporary page" vs "surplus page". It is at the very end of this e-mail. >> When looking at the details, I came up with what may be an even better >> approach. Why not just call the low level routine to free the page instead >> of going through put_page/free_huge_page? At the very least, it saves a >> lock roundtrip and there is no need to worry about the counters/accounting. >> >> Here is a patch to do that. However, we are optimizing a return path in >> a race condition that we are unlikely to ever hit. I 'tested' it by allocating >> an 'extra' page and freeing it via this method in alloc_surplus_huge_page. >> >> >From 864c5f8ef4900c95ca3f6f2363a85f3cb25e793e Mon Sep 17 00:00:00 2001 >> From: Mike Kravetz >> Date: Tue, 11 Aug 2020 12:45:41 -0700 >> Subject: [PATCH] hugetlb: optimize race error return in >> alloc_surplus_huge_page >> >> The routine alloc_surplus_huge_page() could race with with a pool >> size change. If this happens, the allocated page may not be needed. >> To free the page, the current code will 'Abuse temporary page to >> workaround the nasty free_huge_page codeflow'. Instead, directly >> call the low level routine that free_huge_page uses. This works >> out well because the page is new, we hold the only reference and >> already hold the hugetlb_lock. >> >> Signed-off-by: Mike Kravetz >> --- >> mm/hugetlb.c | 13 ++++++++----- >> 1 file changed, 8 insertions(+), 5 deletions(-) >> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index 590111ea6975..ac89b91fba86 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -1923,14 +1923,17 @@ static struct page *alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask, >> /* >> * We could have raced with the pool size change. >> * Double check that and simply deallocate the new page >> - * if we would end up overcommiting the surpluses. Abuse >> - * temporary page to workaround the nasty free_huge_page >> - * codeflow >> + * if we would end up overcommiting the surpluses. >> */ >> if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) { >> - SetPageHugeTemporary(page); >> + /* >> + * Since this page is new, we hold the only reference, and >> + * we already hold the hugetlb_lock call the low level free >> + * page routine. This saves at least a lock roundtrip. >> + */ >> + (void)put_page_testzero(page); /* don't call destructor */ >> + update_and_free_page(h, page); >> spin_unlock(&hugetlb_lock); >> - put_page(page); >> return NULL; >> } else { >> h->surplus_huge_pages++; > >Yes this makes sense. I would have to think about this more to be >confident and give Acked-by but this looks sensible from a quick glance. > If it is ok, I would like to send v2 without this one to give more time for a discussion? >Thanks! >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me